Exception Replay docs major update for new Settings page

[sstonehill12]

What does this PR do? What is the motivation?

Rewrite and reorganizes the Exception Replay documentation for APM Error Tracking to incorporate the platform and functionality changes made in the past couple of quarters, including: New Settings page, in-app enablement, environment-level settings, redaction modes.

Refs: DEBUG-4665

Merge instructions

Merge readiness:

• [ ] Ready for merge

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Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/sarah.stonehill/exception-replay-settings-updates/error_tracking/backend/exception_replay